Mild traumatic brain injury (mTBI), otherwise known as concussion, is a common battlefield injury that, in the military, typically results from exposure to improvised explosive devices (IEDs), falls, vehicular crashes, other combat-related activities, training, or sports. Over 379,000 United States military service members (SMs) sustained a TBI of any severity between 2000 and 2017 (15% due to combat-related activities in Iraq and Afghanistan; 82.3% classified as mTBI). Past studies suggest a link between single and repetitive mTBI/concussion and associated clinical symptoms, including sleep disturbance, headaches, and memory problems. A greater number of total lifetime concussions has been linked to increased insomnia, depression, emotional distress, and concussion symptom severity. Similarly, history of mTBI/concussion has been associated with subsequent development of neurological and psychiatric illness, such as mood disorders, mild cognitive impairment, chronic traumatic encephalopathy, dementia (in particular, Alzheimer?s-type). Because of the severity of symptoms and significant impact on quality of life, early identification of these diseases is imperative. Several biomarkers associated with these diseases have been found in cerebrospinal fluid (CSF); however, CSF is difficult to obtain due to the invasive nature of its collection. Therefore, we wish to collect preliminary data on blood biomarkers in military personnel who have sustained a concussion or blast exposure during combat deployment, in order to compare concentrations post-injury to pre-injury. We hypothesize that changes in neuronal biomarkers will be linked to chronic, persistent clinical symptoms following concussion.